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A departure from the usual assumption of a uniform magnetic field in the calculation of the Harris dispersion relation is shown to lead to new velocity-space instability. If the magnetic field variations are small and if a particle transit time through a complete cycle of the field is less than the instability growth time, an average (velocity-dependent) gyrofrequency may be assumed which leads to negative-mass-type instabilities.
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Clarke et al. (1968) studied this question.
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